The nuclear receptor PXR is a lithocholic acid sensor that protects against liver toxicity

Lycopene prevents DEHP-induced testicular endoplasmic reticulum stress via regulating nuclear xenobiotic receptors and unfolded protein response in mice

Lycopene (LYC) is a potent antioxidant synthesized by red vegetables or plants. Di-2-ethylhexyl phthalate (DEHP) is frequently detected in diverse agricultural environments and considered as a reproductive toxicant. The present research was designed to assess the potential mechanisms of DEHP-induced testicular toxicity and the treatment efficacy of LYC. In this study, after the oral administration of LYC at the dose of 5 mg per kg b.w. per day, mice were given 500 or 1000 mg per kg b.w. per day of DEHP. This research suggested that LYC prevented the DEHP-induced disorder at the levels of activity and content of CYP450 enzymes. LYC attenuated DEHP-caused enhancement in nuclear xenobiotic receptors (NXRs) and the phase I metabolizing enzymes (CYP1, CYP2, CYP3, etc.) levels. Furthermore, endoplasmic reticulum (ER) stress was induced by DEHP and triggered unfolded protein response (UPR). Interestingly, LYC could effectively ameliorate these "hit". The present study suggested that LYC prevents DEHP-induced ER stress in testis via regulating NXRs and UPRER.

Effects of Dicyclohexyl Phthalate Exposure on PXR Activation and Lipid Homeostasis in Mice

BACKGROUND

Exposure to plastic-associated endocrine disrupting chemicals (EDCs) has been associated with an increased risk of cardiovascular disease (CVD) in humans. However, the underlying mechanisms for this association are unclear. Many EDCs have been shown to function as ligands of the nuclear receptor pregnane X receptor (PXR), which functions as xenobiotic sensor but also has pro-atherogenic effects in vivo.

OBJECTIVE

We sought to investigate the contribution of PXR to the adverse effects dicyclohexyl phthalate (DCHP), a widely used phthalate plasticizer, on lipid homeostasis and CVD risk factors.

METHODS

Cell-based assays, primary organoid cultures, and PXR conditional knockout and PXR-humanized mouse models were used to investigate the impact of DCHP exposure on PXR activation and lipid homeostasis in vitro and in vivo. Targeted lipidomics were performed to measure circulating ceramides, novel predictors for CVD.

RESULTS

DCHP was identified as a potent PXR-selective agonist that led to higher plasma cholesterol levels in wild-type mice. DCHP was then demonstrated to activate intestinal PXR to elicit hyperlipidemia by using tissue-specific PXR-deficient mice. Interestingly, DCHP exposure also led to higher circulating ceramides in a PXR-dependent manner. DCHP-mediated PXR activation stimulated the expression of intestinal genes mediating lipogenesis and ceramide synthesis. Given that PXR exhibits considerable species-specific differences in receptor pharmacology, PXR-humanized mice were also used to replicate these findings.

DISCUSSION

Although the adverse health effects of several well-known phthalates have attracted considerable attention, little is known about the potential impact of DCHP on human health. Our studies demonstrate that DCHP activated PXR to induce hypercholesterolemia and ceramide production in mice. These results indicate a potentially important role of PXR in contributing to the deleterious effects of plastic-associated EDCs on cardiovascular health in humans. Testing PXR activation should be considered for risk assessment of phthalates and other EDCs. https://doi.org/10.1289/EHP9262.

Bile acid homeostasis in female mice deficient in Cyp7a1 and Cyp27a1

Bile acids (BAs) are amphipathic molecules important for metabolism of cholesterol, absorption of lipids and lipid soluble vitamins, bile flow, and regulation of gut microbiome. There are over 30 different BA species known to exist in humans and mice, which are endogenous modulators of at least 6 different membrane or nuclear receptors. This diversity of ligands and receptors play important roles in health and disease; however, the full functions of each individual BA in vivo remain unclear. We generated a mouse model lacking the initiating enzymes, CYP7A1 and CYP27A1, in the two main pathways of BA synthesis. Because females are more susceptible to BA related diseases, such as intrahepatic cholestasis of pregnancy, we expanded this model into female mice. The null mice of Cyp7a1 and Cyp27a1 were crossbred to create double knockout (DKO) mice. BA concentrations in female DKO mice had reductions in serum (63%), liver (83%), gallbladder (94%), and small intestine (85%), as compared to WT mice. Despite low BA levels, DKO mice had a similar expression pattern to that of WT mice for genes involved in BA regulation, synthesis, conjugation, and transport. Additionally, through treatment with a synthetic FXR agonist, GW4064, female DKO mice responded to FXR activation similarly to WT mice.

The Interface between Cell Signaling Pathways and Pregnane X Receptor

Highly expressed in the enterohepatic system, pregnane X receptor (PXR, NR1I2) is a well-characterized nuclear receptor (NR) that regulates the expression of genes in the liver and intestines that encode key drug metabolizing enzymes and drug transporter proteins in mammals. The net effect of PXR activation is to increase metabolism and clear drugs and xenobiotics from the body, producing a protective effect and mediating clinically significant drug interaction in patients on combination therapy. The complete understanding of PXR biology is thus important for the development of safe and effective therapeutic strategies. Furthermore, PXR activation is now known to specifically transrepress the inflammatory- and nutrient-signaling pathways of gene expression, thereby providing a mechanism for linking these signaling pathways together with enzymatic drug biotransformation pathways in the liver and intestines. Recent research efforts highlight numerous post-translational modifications (PTMs) which significantly influence the biological function of PXR. However, this thrust of research is still in its infancy. In the context of gene-environment interactions, we present a review of the recent literature that implicates PXR PTMs in regulating its clinically relevant biology. We also provide a discussion of how these PTMs likely interface with each other to respond to extracellular cues to appropriately modify PXR activity.

Regulation of PXR Function by Coactivator and Corepressor Proteins: Ligand Binding Is Just the Beginning

The pregnane X receptor (PXR, NR1I2) is a nuclear receptor which exerts its regulatory function by heterodimerization with the retinoid-X-receptor α (RXRα, NR2B1) and binding to the promoter and enhancer regions of diverse target genes. PXR is involved in the regulation of drug metabolism and excretion, metabolic and immunological functions and cancer pathogenesis. PXR activity is strongly regulated by the association with coactivator and corepressor proteins. Coactivator proteins exhibit histone acetyltransferase or histone methyltransferase activity or associate with proteins having one of these activities, thus promoting chromatin decondensation and activation of the gene expression. On the contrary, corepressor proteins promote histone deacetylation and therefore favor chromatin condensation and repression of the gene expression. Several studies pointed to clear cell- and ligand-specific differences in the activation of PXR. In this article, we will review the critical role of coactivator and corepressor proteins as molecular determinants of the specificity of PXR-mediated effects. As already known for other nuclear receptors, understanding the complex mechanism of PXR activation in each cell type and under particular physiological and pathophysiological conditions may lead to the development of selective modulators with therapeutic potential.

Pregnane X receptor exacerbates nonalcoholic fatty liver disease accompanied by obesity- and inflammation-prone gut microbiome signature

Nonalcoholic fatty liver disease (NAFLD) is the most prevalent chronic liver disease due to the current epidemics of obesity and diabetes. The pregnane X receptor (PXR) is a xenobiotic-sensing nuclear receptor known for trans-activating liver genes involved in drug metabolism and transport, and more recently implicated in energy metabolism. The gut microbiota can modulate the host xenobiotic biotransformation and contribute to the development of obesity. While the male sex confers a higher risk for NAFLD than women before menopause, the mechanism remains unknown. We hypothesized that the presence of PXR promotes obesity by modifying the gut-liver axis in a sex-specific manner. Male and female C57BL/6 (wild-type/WT) and PXR-knockout (PXR-KO) mice were fed control or high-fat diet (HFD) for 16-weeks. Serum parameters, liver histopathology, transcriptomic profiling, 16S-rDNA sequencing, and bile acid (BA) metabolomics were performed. PXR enhanced HFD-induced weight gain, hepatic steatosis and inflammation especially in males, accompanied by PXR-dependent up-regulation in hepatic genes involved in microbial response, inflammation, oxidative stress, and cancer; PXR-dependent increase in intestinal Firmicutes/Bacteroides ratio (hallmark of obesity) and the pro-inflammatory Lactobacillus, as well as a decrease in the anti-obese Allobaculum and the anti-inflammatory Bifidobacterum, with a PXR-dependent reduction of beneficial BAs in liver. The resistance to NAFLD in females may be explained by PXR-dependent decrease in pro-inflammatory bacteria (Ruminococcus gnavus and Peptococcaceae). In conclusion, PXR exacerbates hepatic steatosis and inflammation accompanied by obesity- and inflammation-prone gut microbiome signature, suggesting that gut microbiome may contribute to PXR-mediated exacerbation of NAFLD.

Dietary phytochemicals as modulators of human pregnane X receptor

As a promiscuous xenobiotic sensor, pregnane X receptor (PXR) plays a crucial role in drug metabolism. Since dietary phytochemicals exhibit the potential to modulate human PXR, this review aims to summarize the plant-derived PXR modulators, including agonists, partial agonists, and antagonists. The crystal structures of the apo and ligand-bound forms of PXR especially that of PXR complexed with binary mixtures are summarized, in order to provide the structural basis for PXR binding promiscuity and synergistic activation of PXR by composite ligands. Furthermore, this review summarizes the characterized agonists, partial agonists, and antagonists of human PXR from botanical source. Contrary to PXR agonists, there are only a few antagonists obtained from botanical source due to the promiscuity of PXR. It is worth noting that trans-resveratrol and a series of methylindoles have been identified as partial agonists of PXR, both in activating PXR function, but also inhibiting the effect of other PXR agonists. Since antagonizing PXR function plays a crucial role in the prevention of drug-drug interactions and improvement of therapeutic efficacy, further research is necessary to screen more plant-derived PXR antagonists in the future. In summary, this review may contribute to understanding the roles of phytochemicals in food-drug and herb-drug interactions.

The insecticide chlorpyrifos modifies the expression of genes involved in the PXR and AhR pathways in the rainbow trout, Oncorhynchus mykiss

Chlorpyrifos (CPF) is an organophosphate pesticide, commonly detected in water and food. Despite CPF toxicity on aquatic species has been extensively studied, few studies analyze the effects of CPF on fish transcriptional pathways. The Pregnane X receptor (PXR) is a nuclear receptor that is activated by binding to a wide variety of ligands and regulates the transcription of enzymes involved in the metabolism and transport of many endogenous and exogenous compounds. We evaluated the mRNA expression of PXR-regulated-genes (PXR, CYP3A27, CYP2K1, ABCB1, UGT, and ABCC2) in intestine and liver of the rainbow trout, Oncorhynchus mykiss, exposed in vivo to an environmentally relevant CPF concentration. Our results demonstrate that the expression of PXR and PXR-regulated genes is increased in O. mykiss liver and intestine upon exposure to CPF. Additionally, we evaluated the impact of CPF on other cellular pathway involved in xenobiotic metabolism, the Aryl Hydrocarbon Receptor (AhR) pathway, and on the expression and activity of different biotransformation enzymes (CYP2M1, GST, FMO1, or cholinesterases (ChEs)). In contrast to PXR, the expression of AhR, and its target gene CYP1A, are reduced upon CPF exposure. Furthermore, ChE and CYP1A activities are significantly inhibited by CPF, in both the intestine and the liver. CPF activates the PXR pathway in O. mykiss in the intestine and liver, with a more profound effect in the intestine. Likewise, our results support regulatory crosstalk between PXR and AhR pathways, where the induction of PXR coincides with the downregulation of AhR-mediated CYP1A mRNA expression and activity in the intestine.

Redox-Dependent Effects in the Physiopathological Role of Bile Acids

Bile acids (BA) are recognized by their role in nutrient absorption. However, there is growing evidence that BA also have endocrine and metabolic functions. Besides, the steroidal-derived structure gives BA a toxic potential over the biological membrane. Thus, cholestatic disorders, characterized by elevated BA on the liver and serum, are a significant cause of liver transplant and extrahepatic complications, such as skeletal muscle, central nervous system (CNS), heart, and placenta. Further, the BA have an essential role in cellular damage, mediating processes such as membrane disruption, mitochondrial dysfunction, and the generation of reactive oxygen species (ROS) and oxidative stress. The purpose of this review is to describe the BA and their role on hepatic and extrahepatic complications in cholestatic diseases, focusing on the association between BA and the generation of oxidative stress that mediates tissue damage.

Potential gut-brain mechanisms behind adverse mental health outcomes of bariatric surgery

Bariatric surgery induces sustained weight loss and metabolic benefits via notable effects on the gut-brain axis that lead to alterations in the neuroendocrine regulation of appetite and glycaemia. However, in a subset of patients, bariatric surgery is associated with adverse effects on mental health, including increased risk of suicide or self-harm as well as the emergence of depression and substance use disorders. The contributing factors behind these adverse effects are not well understood. Accumulating evidence indicates that there are important links between gut-derived hormones, microbial and bile acid profiles, and disorders of mood and substance use, which warrant further exploration in the context of changes in gut-brain signalling after bariatric surgery. Understanding the basis of these adverse effects is essential in order to optimize the health and well-being of people undergoing treatment for obesity.

Bile acid activated receptors: Integrating immune and metabolic regulation in non-alcoholic fatty liver disease

Bile acids are a family of atypical steroids generated at the interface of liver-intestinal microbiota acting on a ubiquitously expressed family of membrane and nuclear receptors known as bile acid activated receptors. The two best characterized receptors of this family are the nuclear receptor, farnesoid X receptor (FXR) and the G protein-coupled receptor, G protein-coupled bile acid receptor 1 (GPBAR1). FXR and GPBAR1 regulate major aspects of lipid and glucose metabolism, energy balance, autophagy and immunity and have emerged as potential pharmaceutical targets for the treatment of metabolic and inflammatory disorders. Clinical trials in non-alcoholic fatty liver disease (NAFLD), however, have shown that selective FXR agonists cause side effects while their efficacy is partial. Because FXR and GPBAR1 exert additive effects, dual FXR/GPBAR1 ligands have been developed for the treatment of metabolic disorders and are currently advanced to clinical trials. Here, we will review the role of FXR and GPBAR1 agonism in NAFLD and how the two receptors could be exploited to target multiple components of the disease.

Bile acid receptors and signaling crosstalk in the liver, gut and brain

Bile acids are physiological detergents derived from cholesterol that aid in digestion and nutrient absorption, and they play roles in glucose, lipid, and energy metabolism and in gut microbiome and metabolic homeostasis. Bile acids mediate crosstalk between the liver and gut through bactericidal modulation of the gut microbiome, while gut microbes influence the composition of the circulating bile acid pool. Recent research indicates bile acids may also be important mediators of neurological disease by acting as peripheral signaling molecules that activate bile acid receptors in the blood-brain barrier and in the brain itself. This review highlights the role of bile acids in maintaining liver and gut microbe homeostasis, as well as their function as mediators of cellular signaling in the liver-gut-brain axis.

Vitamin D and the risk for cancer: A molecular analysis

Uncontrolled overgrowth of cells, such as in cancer, is an unavoidable risk in life that affects nearly every second individual in industrialized countries. However, in part this risk can be controlled through lifestyle adjustments, such as the avoidance of smoking, unhealthy diet, obesity, physical inactivity and other cancer risk factors. A low vitaminD status is a risk in particular for cancers of colon, prostate, breast and leukocytes. VitaminD₃ is produced non-enzymatically, when the cholesterol precursor 7-dehydrocholesterol is exposed to UV-B from sunlight, i.e., all cholesterol synthesizing species, including humans, can make vitaminD₃. VitaminD endocrinology started some 550million years ago, when the metabolite 1α,25-dihydroxyvitaminD₃ and the transcription factor vitaminD receptor teamed up for regulating the expression of hundreds of target genes in a multitude of different tissues and cell types. Initially, these genes were focused on the control of energy homeostasis, which later also involved energy-demanding innate and adaptive immunity. Rapidly growing cells of the immune system as well as those of malignant tumors rely on comparable genes and pathways, some of which are modulated by vitaminD. Accordingly, vitaminD has anti-cancer effects both directly via controling the differentiation, proliferation and apoptosis of neoplastic cells as well as indirectly through regulating immune cells that belong to the microenvironment of malignant tumors. This review discusses effects of vitaminD on the epigenome and transcriptome of stromal and tumor cells, inter-individual variations in vitaminD responsiveness and their relation to the prevention and possible therapy of cancer.

Deconvolution of Cytochrome P450 Induction Mechanisms in HepaRG Nuclear Hormone Receptor Knockout Cells

Pregnane X receptor (PXR), constitutive androstane receptor (CAR), and PXR/CAR knockout (KO) HepaRG cells, as well as a PXR reporter gene assay, were used to investigate the mechanism of CYP3A4 and CYP2B6 induction by prototypical substrates and a group of compounds from the Merck KGaA oncology drug discovery pipeline. The basal and inducible gene expression of CYP3A4 and CYP2B6 of nuclear hormone receptor (NHR) KO HepaRG relative to control HepaRG was characterized. The basal expression of CYP3A4 was markedly higher in the PXR (10-fold) and CAR (11-fold) KO cell lines compared with control HepaRG, whereas inducibility was substantially lower. Inversely, basal expression of CYP3A4 in PXR/CAR double KO (dKO) was low (10-fold reduction). Basal CYP2B6 expression was high in PXR KO (9-fold) cells which showed low inducibility, whereas the basal expression remained unchanged in CAR and dKO cell lines compared with control cells. Most of the test compounds induced CYP3A4 and CYP2B6 via PXR and, to a lesser extent, via CAR. Furthermore, other non-NHR-driven induction mechanisms were implicated, either alone or in addition to NHRs. Notably, 5 of the 16 compounds (31%) that were PXR inducers in HepaRG did not activate PXR in the reporter gene assay, illustrating the limitations of this system. This study indicates that HepaRG is a highly sensitive system fit for early screening of cytochrome P450 (P450) induction in drug discovery. Furthermore, it shows the applicability of HepaRG NHR KO cells as tools to deconvolute mechanisms of P450 induction using novel compounds representative for oncology drug discovery. SIGNIFICANCE STATEMENT: This work describes the identification of induction mechanisms of CYP3A4 and CYP2B6 for an assembly of oncology drug candidates using HepaRG nuclear hormone receptor knockout and displays its advantages compared to a pregnane X receptor reporter gene assay. With this study, risk assessment of drug candidates in early drug development can be improved.

Negative Regulation of Human Hepatic Constitutive Androstane Receptor by Cholesterol Synthesis Inhibition: Role of Sterol Regulatory Element Binding Proteins

The squalene synthase inhibitor squalestatin 1 (Squal1) is a potent and efficacious inducer of CYP2B expression in primary cultured rat hepatocytes and rat liver. To determine whether Squal1 is also an inducer of human CYP2B, the effects of Squal1 treatment were evaluated in primary cultured human hepatocytes, differentiated HepaRG cells, and humanized mouse livers. Squal1 treatment did not increase CYP2B6 mRNA levels in human hepatocytes or HepaRG cells and only slightly and inconsistently increased CYP2B6 mRNA content in humanized mouse liver. However, treatment with farnesol, which mediates Squal1's effect on rat CYP2B expression, increased CYP2B6 mRNA levels in HepaRG cells expressing the constitutive androstane receptor (CAR), but not in cells with knocked-down CAR. To determine the impact of cholesterol biosynthesis inhibition on CAR activation, the effects of pravastatin (Prava) were determined on CITCO-mediated gene expression in primary cultured human hepatocytes. Prava treatment abolished CITCO-inducible CYP2B6 expression, but had less effect on rifampicin-mediated CYP3A4 induction, and CITCO treatment did not affect Prava-inducible HMG-CoA reductase (HMGCR) expression. Treatment with inhibitors of different steps of cholesterol biosynthesis attenuated CITCO-mediated CYP2B6 induction in HepaRG cells, and Prava treatment increased HMGCR expression and inhibited CYP2B6 induction with comparable potency. Transfection of HepG2 cells with transcriptionally active sterol regulatory element binding proteins (SREBPs) reduced CAR-mediated transactivation, and inducible expression of transcriptionally active SREBP2 attenuated CITCO-inducible CYP2B6 expression in HepaRG cells. These findings suggest that Squal1 does not induce CYP2B6 in human hepatocytes because Squal1's inhibitory effect on cholesterol biosynthesis interferes with CAR activation. SIGNIFICANCE STATEMENT: The cholesterol biosynthesis inhibitor squalestatin 1 induces rat hepatic CYP2B expression indirectly by causing accumulation of an endogenous isoprenoid that activates the constitutive androstane receptor (CAR). This study demonstrates that squalestatin 1 does not similarly induce CYP2B6 expression in human hepatocytes. Rather, inhibition of cholesterol biosynthesis interferes with CAR activity, likely by activating sterol regulatory element binding proteins. These findings increase our understanding of the endogenous processes that modulate human drug-metabolizing gene expression.

The Role of Vitamin K in Cholestatic Liver Disease

Vitamin K (VK) is a ligand of the pregnane X receptor (PXR), which plays a critical role in the detoxification of xenobiotics and metabolism of bile acids. VK₁ may reduce the risk of death in patients with chronic liver failure. VK deficiency is associated with intrahepatic cholestasis, and is already being used as a drug for cholestasis-induced liver fibrosis in China. In Japan, to treat osteoporosis in patients with primary biliary cholangitis, VK₂ formulations are prescribed, along with vitamin D₃. Animal studies have revealed that after bile duct ligation-induced cholestasis, PXR knockout mice manifested more hepatic damage than wild-type mice. Ligand-mediated activation of PXR improves biochemical parameters. Rifampicin is a well-known human PXR ligand that has been used to treat intractable pruritus in severe cholestasis. In addition to its anti-cholestatic properties, PXR has anti-fibrotic and anti-inflammatory effects. However, because of the scarcity of animal studies, the mechanism of the effect of VK on cholestasis-related liver disease has not yet been revealed. Moreover, the application of VK in cholestasis-related diseases is controversial. Considering this background, the present review focuses on the effect of VK in cholestasis-related diseases, emphasizing its function as a modulator of PXR.

Lithocholic Acid Activates Mas-Related G Protein-Coupled Receptors, Contributing to Itch in Mice

The present study focused on lithocholic acid (LCA), a secondary bile acid that contributes to cholestatic pruritus. Although recent studies have found that LCA acts on MAS-related G protein-coupled receptor family member X4 (MRGPRX4) in humans, it is unclear which subtypes of MRGPRs are activated by LCA in mice since there is no precise ortholog of human MRGPRX4 in the mouse genome. Using calcium imaging, we found that LCA could activate mouse Mrgpra1 when transiently expressed in HEK293T cells. Moreover, LCA similarly activates mouse Mrgprb2. Importantly, LCA-induced responses showed dose-dependent effects through Mrgpra1 and Mrgprb2. Moreover, treatment with QWF (an antagonist of Mrgpra1 and Mrgprb2), YM254890 (Gα_q inhibitor), and U73122 (an inhibitor of phospholipase C) significantly suppressed the LCA-induced responses, implying that the LCA-induced responses are indeed mediated by Mrgpra1 and Mrgprb2. Furthermore, LCA activated primary cultures of mouse sensory neurons and peritoneal mast cells, suggesting that Mrgpra1 and Mrgprb2 contribute to LCA-induced pruritus. However, acute injection of LCA did not induce noticeable differences in scratching behavior, implying that the pruritogenic role of LCA may be marginal in non-cholestatic conditions. In summary, the present study identified for the first time that LCA can activate Mrgpra1 and Mrgprb2. The current findings provide further insight into the similarities and differences between human and mouse MRGPR families, paving a way to understand the complex roles of these pruriceptors.

Lathyrane Diterpenoids as Novel hPXR Agonists: Isolation, Structural Modification, and Structure-Activity Relationships

Pregnane X receptor (PXR) that orchestrates the intricate network of xeno- and endobiotic metabolism is considered as a promising therapeutic target for cholestasis. In this study, the human PXR (hPXR) agonistic bioassay-guided isolation of Euphorbia lathyris followed by the structural modification led to the construction of a lathyrane diterpenoid library (1-34). Subsequent assay of this library led to the identification of a series of potent hPXR agonists, showing better efficacy than that of typical hPXR agonist, rifampicin. The most active compound, 8, could dose-dependently activate hPXR at micromolar concentrations and significantly up-regulate the expressions of PXR downstream genes CYP3A4, CYP2B6, and MDR1. The structure-activity relationships (SARs) studied in combination with molecular modeling suggested that acyloxy at C-7 and the presence of 14-carbonyl were essential to the activity. These findings suggested that lathyrane diterpenoids could serve as a new type of hPXR agonist for future anticholestasis drug development.

Critical roles of bile acids in regulating intestinal mucosal immune responses

Bile acids are a class of cholesterol derivatives that have been known for a long time for their critical roles in facilitating the digestion and absorption of lipid from the daily diet. The transformation of primary bile acids produced by the liver to secondary bile acids appears under the action of microbiota in the intestine, greatly expanding the molecular diversity of the intestinal environment. With the discovery of several new receptors of bile acids and signaling pathways, bile acids are considered as a family of important metabolites that play pleiotropic roles in regulating many aspects of human overall health, especially in the maintenance of the microbiota homeostasis and the balance of the mucosal immune system in the intestine. Accordingly, disruption of the process involved in the metabolism or circulation of bile acids is implicated in many disorders that mainly affect the intestine, such as inflammatory bowel disease and colon cancer. In this review, we discuss the different metabolism profiles in diseases associated with the intestinal mucosa and the diverse roles of bile acids in regulating the intestinal immune system. Furthermore, we also summarize recent advances in the field of new drugs that target bile acid signaling and highlight the importance of bile acids as a new target for disease intervention.

Bile Acids Activated Receptors in Inflammatory Bowel Disease

Once known exclusively for their role in nutrients absorption, bile acids have emerged as signaling molecules, generated from cholesterol breakdown, acting on several immune cells by activating a variety of receptors including the G protein-coupled bile acid receptor 1 (GPABR1 or TGR5), the Farnesoid-X-receptor (FXR) and, as recently discovered, the retinoid-related orphan receptors (ROR)γt. GPBAR1, FXR, and RORγt are highly expressed in cells of the innate and adaptive immune system (i.e., dendritic cells (DCs), macrophages, innate lymphoid 3 cells (ILC3s), and T helper 17 (Th17) lymphocytes) and plays an important role in regulating intestinal and liver immunity, highlighting a role for various bile acid species in regulating immune responses to intestinal microbial antigens. While primary bile acids are generated from the cholesterol breakdown secondary bile acids, the GPBAR1 ligands, and oxo-bile acids derivatives, the RORγt ligands, are generated by the intestinal microbiota, highlighting the potential of these bile acids in mediating the chemical communication between the intestinal microbiota and the host. Changes in intestinal microbiota, dysbiosis, alter the composition of the bile acid pool, promoting the activation of the immune system and development of chronic inflammation. In this review, we focus on the molecular mechanisms by which an altered bile acid signaling promotes intestinal inflammation.

Effect of Vitamin K-Mediated PXR Activation on Drug-Metabolizing Gene Expression in Human Intestinal Carcinoma LS180 Cell Line

The pregnane X receptor (PXR) is the key regulator of our defense mechanism against foreign substances such as drugs, dietary nutrients, or environmental pollutants. Because of increased health consciousness, the use of dietary supplements has gradually increased, and most of them can activate PXR. Therefore, an analysis of the interaction between drugs and nutrients is important because altered levels of drug-metabolizing enzymes or transporters can remarkably affect the efficiency of a co-administered drug. In the present study, we analyzed the effect of vitamin K-mediated PXR activation on drug metabolism-related gene expression in intestine-derived LS180 cells via gene expression studies and western blotting analyses. We demonstrated that menaquinone 4 (MK-4), along with other vitamin Ks, including vitamin K₁, has the potential to induce MDR1 and CYP3A4 gene expression. We showed that PXR knockdown reversed MK-4-mediated stimulation of these genes, indicating the involvement of PXR in this effect. In addition, we showed that the expression of MDR1 and CYP3A4 genes increased synergistically after 24 h of rifampicin and MK-4 co-treatment. Our study thus elucidates the importance of drug–nutrient interaction mediated via PXR.

The Impact of the Microbiome on Immunity to Vaccination in Humans

Vaccines are the most effective means available for preventing infectious diseases. However, vaccine-induced immune responses are highly variable between individuals and between populations in different regions of the world. Understanding the basis of this variation is, thus, of fundamental importance to human health. Although the factors that are associated with intra- and inter-population variation in vaccine responses are manifold, emerging evidence points to a key role for the gut microbiome in controlling immune responses to vaccination. Much of this evidence comes from studies in mice, and causal evidence for the impact of the microbiome on human immunity is sparse. However, recent studies on vaccination in subjects treated with broad-spectrum antibiotics have provided causal evidence and mechanistic insights into how the microbiota controls immune responses in humans.

Contribution of the Gut Microbiome to Drug Disposition, Pharmacokinetic and Pharmacodynamic Variability

The trillions of microbes that make up the gut microbiome are an important contributor to health and disease. With respect to xenobiotics, particularly orally administered compounds, the gut microbiome interacts directly with drugs to break them down into metabolic products. In addition, microbial products such as bile acids interact with nuclear receptors on host drug-metabolizing enzyme machinery, thus indirectly influencing drug disposition and pharmacokinetics. Gut microbes also influence drugs that undergo enterohepatic recycling by reversing host enzyme metabolic processes and increasing exposure to toxic metabolites as exemplified by the chemotherapy agent irinotecan and non-steroidal anti-inflammatory drugs. Recent data with immune checkpoint inhibitors demonstrate the impact of the gut microbiome on drug pharmacodynamics. We summarize the clinical importance of gut microbe interaction with digoxin, irinotecan, immune checkpoint inhibitors, levodopa, and non-steroidal anti-inflammatory drugs. Understanding the complex interactions of the gut microbiome with xenobiotics is challenging; and highly sensitive methods such as untargeted metabolomics with molecular networking along with other in silico methods and animal and human in vivo studies will uncover mechanisms and pathways. Incorporating the contribution of the gut microbiome to drug disposition, pharmacokinetics, and pharmacodynamics is vital in this era of precision medicine.

Xanthohumol ameliorates Diet-Induced Liver Dysfunction via Farnesoid X Receptor-Dependent and Independent Signaling

The farnesoid X receptor (FXR) plays a critical role in the regulation of lipid and bile acid (BA) homeostasis. Hepatic FXR loss results in lipid and BA accumulation, and progression from hepatic steatosis to nonalcoholic steatohepatitis (NASH). This study aimed to evaluate the effects of xanthohumol (XN), a hop-derived compound mitigating metabolic syndrome, on liver damage induced by diet and FXR deficiency in mice. Wild-type (WT) and liver-specific FXR-null mice (FXR^Liver−/−) were fed a high-fat diet (HFD) containing XN or the vehicle formation followed by histological characterization, lipid, BA and gene profiling. HFD supplemented with XN resulted in amelioration of hepatic steatosis and decreased BA concentrations in FXR^Liver−/− mice, the effect being stronger in male mice. XN induced the constitutive androstane receptor (CAR), pregnane X receptor (PXR) and glucocorticoid receptor (GR) gene expression in the liver of FXR^Liver−/− mice. These findings suggest that activation of BA detoxification pathways represents the predominant mechanism for controlling hydrophobic BA concentrations in FXR^Liver−/− mice. Collectively, these data indicated sex-dependent relationship between FXR, lipids and BAs, and suggest that XN ameliorates HFD-induced liver dysfunction via FXR-dependent and independent signaling.

The pregnane X receptor drives sexually dimorphic hepatic changes in lipid and xenobiotic metabolism in response to gut microbiota in mice

BACKGROUND

The gut microbiota-intestine-liver relationship is emerging as an important factor in multiple hepatic pathologies, but the hepatic sensors and effectors of microbial signals are not well defined.

RESULTS

By comparing publicly available liver transcriptomics data from conventional vs. germ-free mice, we identified pregnane X receptor (PXR, NR1I2) transcriptional activity as strongly affected by the absence of gut microbes. Microbiota depletion using antibiotics in Pxr^+/+ vs Pxr^-/- C57BL/6J littermate mice followed by hepatic transcriptomics revealed that most microbiota-sensitive genes were PXR-dependent in the liver in males, but not in females. Pathway enrichment analysis suggested that microbiota-PXR interaction controlled fatty acid and xenobiotic metabolism. We confirmed that antibiotic treatment reduced liver triglyceride content and hampered xenobiotic metabolism in the liver from Pxr^+/+ but not Pxr^-/- male mice.

CONCLUSIONS

These findings identify PXR as a hepatic effector of microbiota-derived signals that regulate the host's sexually dimorphic lipid and xenobiotic metabolisms in the liver. Thus, our results reveal a potential new mechanism for unexpected drug-drug or food-drug interactions. Video abstract.

CAR directs T cell adaptation to bile acids in the small intestine

Bile acids are lipid-emulsifying metabolites synthesized in hepatocytes and maintained in vivo through enterohepatic circulation between the liver and small intestine¹. As detergents, bile acids can cause toxicity and inflammation in enterohepatic tissues². Nuclear receptors maintain bile acid homeostasis in hepatocytes and enterocytes³, but it is unclear how mucosal immune cells tolerate high concentrations of bile acids in the small intestine lamina propria (siLP). CD4⁺ T effector (T_eff) cells upregulate expression of the xenobiotic transporter MDR1 (encoded by Abcb1a) in the siLP to prevent bile acid toxicity and suppress Crohn's disease-like small bowel inflammation⁴. Here we identify the nuclear xenobiotic receptor CAR (encoded by Nr1i3) as a regulator of MDR1 expression in T cells that can safeguard against bile acid toxicity and inflammation in the mouse small intestine. Activation of CAR induced large-scale transcriptional reprogramming in T_eff cells that infiltrated the siLP, but not the colon. CAR induced the expression of not only detoxifying enzymes and transporters in siLP T_eff cells, as in hepatocytes, but also the key anti-inflammatory cytokine IL-10. Accordingly, CAR deficiency in T cells exacerbated bile acid-driven ileitis in T cell-reconstituted Rag1^-/- or Rag2^-/- mice, whereas pharmacological activation of CAR suppressed it. These data suggest that CAR acts locally in T cells that infiltrate the small intestine to detoxify bile acids and resolve inflammation. Activation of this program offers an unexpected strategy to treat small bowel Crohn's disease and defines lymphocyte sub-specialization in the small intestine.

Nonclinical toxicology studies with sodium taurodeoxycholate: acute and subacute toxicity in dogs

Sodium taurodeoxycholate (TDCA) has been investigated for various inflammatory disorders such as sepsis. We recently evaluated nonclinical safety profile of TDCA using rats infused intravenously. As a series of preclinical safety investigations, we further conducted toxicity studies with TDCA delivered to dogs via intravenous administration under Good Laboratory Practice regulation in this study. In dose range-finding study (dose escalation study), dogs given with TDCA at a dose of 150 mg/kg showed marked changes in clinical signs, hematology, and serum biochemistry. And biochemical markers of liver damage and local skin lesions were observed following intravenous infusion of 100 mg/kg TDCA, suggesting that 100 mg/kg was chosen as the highest dose of TDCA for 4-week repeated-dose toxicity study using dogs. Despite no treatment-related significant changes in body weight, food consumption, ophthalmoscopy, and urinalysis, skin lesions were observed at the injection site of animals administered with higher than 50 mg/kg of TDCA along with biochemical and histopathological changes associated with liver injury. However, most of off-target effects were found to be reversible since these were recovered after stopping TDCA infusion. These findings indicate that the no-observed-adverse-effect-level (NOAEL) for TDCA in dogs was considered to be 5 mg/kg/d. Taken together, our results provide important toxicological profiles regarding the safe dose of TDCA for drug development or clinical application.

Microbial Hydroxysteroid Dehydrogenases: From Alpha to Omega

Bile acids (BAs) and glucocorticoids are steroid hormones derived from cholesterol that are important signaling molecules in humans and other vertebrates. Hydroxysteroid dehydrogenases (HSDHs) are encoded both by the host and by their resident gut microbiota, and they reversibly convert steroid hydroxyl groups to keto groups. Pairs of HSDHs can reversibly epimerize steroids from α-hydroxy conformations to β-hydroxy, or β-hydroxy to ω-hydroxy in the case of ω-muricholic acid. These reactions often result in products with drastically different physicochemical properties than their precursors, which can result in steroids being activators or inhibitors of host receptors, can affect solubility in fecal water, and can modulate toxicity. Microbial HSDHs modulate sterols associated with diseases such as colorectal cancer, liver cancer, prostate cancer, and polycystic ovary syndrome. Although the role of microbial HSDHs is not yet fully elucidated, they may have therapeutic potential as steroid pool modulators or druggable targets in the future. In this review, we explore metabolism of BAs and glucocorticoids with a focus on biotransformation by microbial HSDHs.

Bile acids and their receptors in metabolic disorders

Bile acids are a large family of atypical steroids which exert their functions by binding to a family of ubiquitous cell membrane and nuclear receptors. There are two main bile acid activated receptors, FXR and GPBAR1, that are exclusively activated by bile acids, while other receptors CAR, LXRs, PXR, RORγT, S1PR2and VDR are activated by bile acids in addition to other more selective endogenous ligands. In the intestine, activation of FXR and GPBAR1 promotes the release of FGF15/19 and GLP1 which integrate their signaling with direct effects exerted by theother receptors in target tissues. This network is tuned in a time ordered manner by circadian rhythm and is critical for the regulation of metabolic process including autophagy, fast-to-feed transition, lipid and glucose metabolism, energy balance and immune responses. In the last decade FXR ligands have entered clinical trials but development of systemic FXR agonists has been proven challenging because their side effects including increased levels of cholesterol and Low Density Lipoproteins cholesterol (LDL-c) and reduced High-Density Lipoprotein cholesterol (HDL-c). In addition, pruritus has emerged as a common, dose related, side effect of FXR ligands. Intestinal-restricted FXR and GPBAR1 agonists and dual FXR/GPBAR1 agonists have been developed. Here we review the last decade in bile acids physiology and pharmacology.

The Liver under the Spotlight: Bile Acids and Oxysterols as Pivotal Actors Controlling Metabolism

Among the myriad of molecules produced by the liver, both bile acids and their precursors, the oxysterols are becoming pivotal bioactive lipids which have been underestimated for a long time. Their actions are ranging from regulation of energy homeostasis (i.e., glucose and lipid metabolism) to inflammation and immunity, thereby opening the avenue to new treatments to tackle metabolic disorders associated with obesity (e.g., type 2 diabetes and hepatic steatosis) and inflammatory diseases. Here, we review the biosynthesis of these endocrine factors including their interconnection with the gut microbiota and their impact on host homeostasis as well as their attractive potential for the development of therapeutic strategies for metabolic disorders.

The xenobiotic receptors PXR and CAR in liver physiology, an update

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are two nuclear receptors that are well-known for their roles in xenobiotic detoxification by regulating the expression of drug-metabolizing enzymes and transporters. In addition to metabolizing drugs and other xenobiotics, the same enzymes and transporters are also responsible for the production and elimination of numerous endogenous chemicals, or endobiotics. Moreover, both PXR and CAR are highly expressed in the liver. As such, it is conceivable that PXR and CAR have major potentials to affect the pathophysiology of the liver by regulating the homeostasis of endobiotics. In recent years, the physiological functions of PXR and CAR in the liver have been extensively studied. Emerging evidence has suggested the roles of PXR and CAR in energy metabolism, bile acid homeostasis, cell proliferation, to name a few. This review summarizes the recent progress in our understanding of the roles of PXR and CAR in liver physiology.

The Gut Microbial Endocrine Organ in Type 2 Diabetes

Historically, the focus of type II diabetes mellitus (T2DM) research has been on host metabolism and hormone action. However, emerging evidence suggests that the gut microbiome, commensal microbes that colonize the gastrointestinal tract, also play a significant role in T2DM pathogenesis. Specifically, gut microbes metabolize what is available to them through the host diet to produce small molecule metabolites that can have endocrine-like effects on human cells. In fact, the meta-organismal crosstalk between gut microbe-generated metabolites and host receptor systems may represent an untapped therapeutic target for those at risk for or suffering from T2DM. Recent evidence suggests that gut microbe-derived metabolites can impact host adiposity, insulin resistance, and hormone secretion to collectively impact T2DM progression. Here we review the current evidence that structurally diverse gut microbe-derived metabolites, including short chain fatty acids, secondary bile acids, aromatic metabolites, trimethylamine-N-oxide, polyamines, and N-acyl amides, that can engage with host receptors in an endocrine-like manner to promote host metabolic disturbance associated with T2DM. Although these microbe-host signaling circuits are not as well understood as host hormonal signaling, they hold untapped potential as new druggable targets to improve T2DM complications. Whether drugs that selectively target meta-organismal endocrinology will be safe and efficacious in treating T2DM is a key new question in the field of endocrinology. Here we discuss the opportunities and challenges in targeting the gut microbial endocrine organ for the treatment of diabetes and potentially many other diseases where diet-microbe-host interactions play a contributory role.

Regulation of Gastrointestinal Immunity by Metabolites

The gastrointestinal tract contains multiple types of immune cells that maintain the balance between tolerance and activation at the first line of host defense facing non-self antigens, including dietary antigens, commensal bacteria, and sometimes unexpected pathogens. The maintenance of homeostasis at the gastrointestinal tract requires stringent regulation of immune responses against various environmental conditions. Dietary components can be converted into gut metabolites with unique functional activities through host as well as microbial enzymatic activities. Accumulating evidence demonstrates that gastrointestinal metabolites have significant impacts on the regulation of intestinal immunity and are further integrated into the immune response of distal mucosal tissue. Metabolites, especially those derived from the microbiota, regulate immune cell functions in various ways, including the recognition and activation of cell surface receptors, the control of gene expression by epigenetic regulation, and the integration of cellular metabolism. These mucosal immune regulations are key to understanding the mechanisms underlying the development of gastrointestinal disorders. Here, we review recent advancements in our understanding of the role of gut metabolites in the regulation of gastrointestinal immunity, highlighting the cellular and molecular regulatory mechanisms by macronutrient-derived metabolites.

Role of Gut Microbiota and Their Metabolites on Atherosclerosis, Hypertension and Human Blood Platelet Function: A Review

Emerging data have demonstrated a strong association between the gut microbiota and the development of cardiovascular disease (CVD) risk factors such as atherosclerosis, inflammation, obesity, insulin resistance, platelet hyperactivity, and plasma lipid abnormalities. Several studies in humans and animal models have demonstrated an association between gut microbial metabolites such as trimethylamine-N-oxide (TMAO), short-chain fatty acids, and bile acid metabolites (amino acid breakdown products) with CVD. Human blood platelets are a critical contributor to the hemostatic process. Besides, these blood cells play a crucial role in developing atherosclerosis and, finally, contribute to cardiac events. Since the TMAO, and other metabolites of the gut microbiota, are asociated with platelet hyperactivity, lipid disorders, and oxidative stress, the diet-gut microbiota interactions have become an important research area in the cardiovascular field. The gut microbiota and their metabolites may be targeted for the therapeutic benefit of CVD from a clinical perspective. This review's main aim is to highlight the complex interactions between microbiota, their metabolites, and several CVD risk factors.

Bile Acid Signaling in Inflammatory Bowel Diseases

Bile acids are a group of chemically different steroids generated at the host/microbial interface. Indeed, while primary bile acids are the end-product of cholesterol breakdown in the host liver, secondary bile acids are the products of microbial metabolism. Primary and secondary bile acids along with their oxo derivatives have been identified as signaling molecules acting on a family of cell membrane and nuclear receptors collectively known as "bile acid-activated receptors." Members of this group of receptors are highly expressed throughout the gastrointestinal tract and mediate the bilateral communications of the intestinal microbiota with the host immune system. The expression and function of bile acid-activated receptors FXR, GPBAR1, PXR, VDR, and RORγt are highly dependent on the structure of the intestinal microbiota and negatively regulated by intestinal inflammation. Studies from gene ablated mice have demonstrated that FXR and GPBAR1 are essential to maintain a tolerogenic phenotype in the intestine, and their ablation promotes the polarization of intestinal T cells and macrophages toward a pro-inflammatory phenotype. RORγt inhibition by oxo-bile acids is essential to constrain Th17 polarization of intestinal lymphocytes. Gene-wide association studies and functional characterizations suggest a potential role for impaired bile acid signaling in development inflammatory bowel diseases (IBD). In this review, we will focus on how bile acids and their receptors mediate communications of intestinal microbiota with the intestinal immune system, describing dynamic changes of bile acid metabolism in IBD and the potential therapeutic application of targeting bile acid signaling in these disorders.

The gut bacterium Extibacter muris produces secondary bile acids and influences liver physiology in gnotobiotic mice

Extibacter muris is a newly described mouse gut bacterium which metabolizes cholic acid (CA) to deoxycholic acid (DCA) via 7α-dehydroxylation. Although bile acids influence metabolic and inflammatory responses, few in vivo models exist for studying their metabolism and impact on the host. Mice were colonized from birth with the simplified community Oligo-MM12 with or without E. muris. As the metabolism of bile acids is known to affect lipid homeostasis, mice were fed either a low- or high-fat diet for eight weeks before sampling and analyses targeting the gut and liver. Multiple Oligo-MM12 strains were capable of deconjugating primary bile acids in vitro. E. muris produced DCA from CA either as pure compound or in mouse bile. This production was inducible by CA in vitro. Ursodeoxycholic, chenodeoxycholic, and β-muricholic acid were not metabolized under the conditions tested. All gnotobiotic mice were stably colonized with E. muris, which showed higher relative abundances after HF diet feeding. The presence of E. muris had minor, diet-dependent effects on Oligo-MM12 communities. The secondary bile acids DCA and surprisingly LCA and their taurine conjugates were detected exclusively in E. muris-colonized mice. E. muris colonization did not influence body weight, white adipose tissue mass, liver histopathology, hepatic aspartate aminotransferase, or blood levels of cholesterol, insulin, and paralytic peptide (PP). However, proteomics revealed shifts in hepatic pathways involved in amino acid, glucose, lipid, energy, and drug metabolism in E. muris-colonized mice. Liver fatty acid composition was substantially altered by dietary fat but not by E. muris.In summary, E. muris stably colonized the gut of mice harboring a simplified community and produced secondary bile acids, which affected proteomes in the liver. This new gnotobiotic mouse model can now be used to study the pathophysiological role of secondary bile acids in vivo.

Distinct Postprandial Bile Acids Responses to a High-Calorie Diet in Men Volunteers Underscore Metabolically Healthy and Unhealthy Phenotypes

Bile acids (BAs) regulate dietary lipid hydrolysis and absorption in the proximal intestine. Several studies have highlighted a determinant role of circulating levels and/or metabolism of BAs in the pathogenesis of major cardiometabolic diseases. Whether changes in BA profiles are causative or are consequence of these diseases remains to be determined. Healthy male volunteers (n = 71) underwent a postprandial exploration following consumption of a hypercaloric high fat typical Western meal providing 1200 kcal. We investigated variations of circulating levels of 28 BA species, together with BA synthesis marker 7α-hydroxy-4-cholesten-3-one (C4) over an approximately diurnal 12 h period. Analysis of BA variations during the postprandial time course revealed two major phenotypes with opposite fluctuations, i.e., circulating levels of each individual species of unconjugated BAs were reduced after meal consumption whereas those of tauro- and glyco-conjugated BAs were increased. By an unbiased classification strategy based on absolute postprandial changes in BA species levels, we classified subjects into three distinct clusters; the two extreme clusters being characterized by the smallest absolute changes in either unconjugated-BAs or conjugated-BAs. Finally, we demonstrated that our clustering based on postprandial changes in BA profiles was associated with specific clinical and biochemical features, including postprandial triglyceride levels, BMI or waist circumference. Altogether, our study reveals that postprandial profiles/patterns of BAs in response to a hypercaloric high fat challenge is associated with healthy or unhealthy metabolic phenotypes that may help in the early identification of subjects at risk of developing metabolic disorders.

PXR and 4β-Hydroxycholesterol Axis and the Components of Metabolic Syndrome

Pregnane X receptor (PXR) activation has been found to regulate glucose and lipid metabolism and affect obesity in response to high-fat diets. PXR also modulates vascular tone. In fact, PXR appears to regulate multiple components of metabolic syndrome. In most cases, the effect of PXR action is harmful to metabolic health, and PXR can be hypothesized to play an important role in metabolic disruption elicited by exposure to endocrine-disrupting chemicals. The majority of the data on the effects of PXR activation on metabolic health come from animal and cell culture experiments. However, randomized, placebo-controlled, human trials indicate that the treatment with PXR ligands impairs glucose tolerance and increases 24-h blood pressure and heart rate. In addition, plasma 4β-hydroxycholesterol (4βHC), formed under the control of PXR in the liver, is associated with lower blood pressure in healthy volunteers. Furthermore, 4βHC regulates cholesterol transporters in peripheral tissues and may activate the beneficial reverse HDL cholesterol transport. In this review, we discuss the current knowledge on the role of PXR and the PXR–4βHC axis in the regulation of components of metabolic syndrome.

Interactions Between Therapeutics for Metabolic Disease, Cardiovascular Risk Factors, and Gut Microbiota

With improved standards of living, the incidence of multiple metabolic disorders has increased year by year, especially major risk factors for cardiovascular disease such as hyperglycemia and hyperlipidemia, continues to increase. Emerging epidemiological data and clinical trials have shown the additional protective effects of some metabolic therapy drugs against cardiovascular diseases. A series of studies have found that these drugs may work by modulating the composition of gut microbiota. In this review, we provide a brief overview of the contribution of the gut microbiota to both metabolic disorders and cardiovascular diseases, as well as the response of gut microbiota to metabolic therapy drugs with cardiovascular benefits. In this manner, we link the recent advances in microbiome studies on metabolic treatment drugs with their cardiovascular protective effects, suggesting that intestinal microorganisms may play a potential role in reducing cardiovascular risk factors. We also discuss the potential of microorganism-targeted therapeutics as treatment strategies for preventing and/or treating cardiovascular disease and highlight the need to establish causal links between therapeutics for metabolic diseases, gut microbiota modulation, and cardiovascular protection.

Bile Acids: A Communication Channel in the Gut-Brain Axis

Bile acids are signalling hormones involved in the regulation of several metabolic pathways. The ability of bile acids to bind and signal through their receptors is modulated by the gut microbiome, since the microbiome contributes to the regulation and synthesis of bile acids as well to their physiochemical properties. From the gut, bacteria have been shown to send signals to the central nervous system via their metabolites, thus affecting the behaviour and brain function of the host organism. In the last years it has become increasingly evident that bile acids affect brain function, during normal physiological and pathological conditions. Although bile acids may be synthesized locally in the brain, the majority of brain bile acids are taken up from the systemic circulation. Since the composition of the brain bile acid pool may be regulated by the action of intestinal bacteria, it is possible that bile acids function as a communication bridge between the gut microbiome and the brain. However, little is known about the molecular mechanisms and the physiological roles of bile acids in the central nervous system. The possibility that bile acids may be a direct link between the intestinal microbiome and the brain is also an understudied subject. Here we review the influence of gut bacteria on the bile acid pool composition and properties, as well as striking evidence showing the role of bile acids as neuroactive molecules.

Associations between Pregnane X Receptor and Breast Cancer Growth and Progression

Pregnane X receptor (PXR, NR1I2) is a member of the ligand-activated nuclear receptor superfamily. This receptor is promiscuous in its activation profile and is responsive to a broad array of both endobiotic and xenobiotic ligands. PXR is involved in pivotal cellular detoxification processes to include the regulation of genes that encode key drug-metabolizing cytochrome-P450 enzymes, oxidative stress response, as well as enzymes that drive steroid and bile acid metabolism. While PXR clearly has important regulatory roles in the liver and gastrointestinal tract, this nuclear receptor also has biological functions in breast tissue. In this review, we highlight current knowledge of PXR’s role in mammary tumor carcinogenesis. The elevated level of PXR expression in cancerous breast tissue suggests a likely interface between aberrant cell division and xeno-protection in cancer cells. Moreover, PXR itself exerts positive effect on the cell cycle, thereby predisposing tumor cells to unchecked proliferation. Activation of PXR also plays a key role in regulating apoptosis, as well as in acquired resistance to chemotherapeutic agents. The repressive role of PXR in regulating inflammatory mediators along with the existence of genetic polymorphisms within the sequence of the PXR gene may predispose individuals to developing breast cancer. Further investigations into the role that PXR plays in driving tumorigenesis are needed.

12α-Hydroxylated bile acid induces hepatic steatosis with dysbiosis in rats

There is an increasing need to explore the mechanism of the progression of non-alcoholic fatty liver disease. Steroid metabolism is closely linked to hepatic steatosis and steroids are excreted as bile acids (BAs). Here, we demonstrated that feeding WKAH/HkmSlc inbred rats a diet supplemented with cholic acid (CA) at 0.5 g/kg for 13 weeks induced simple steatosis without obesity. Liver triglyceride and cholesterol levels were increased accompanied by mild elevation of aminotransferase activities. There were no signs of inflammation, insulin resistance, oxidative stress, or fibrosis. CA supplementation increased levels of CA and taurocholic acid (TCA) in enterohepatic circulation and deoxycholic acid (DCA) levels in cecum with an increased ratio of 12α-hydroxylated BAs to non-12α-hydroxylated BAs. Analyses of hepatic gene expression revealed no apparent feedback control of BA and cholesterol biosynthesis. CA feeding induced dysbiosis in cecal microbiota with enrichment of DCA producers, which underlines the increased cecal DCA levels. The mechanism of steatosis was increased expression of Srebp1 (positive regulator of liver lipogenesis) through activation of the liver X receptor by increased oxysterols in the CA-fed rats, especially 4β-hydroxycholesterol (4βOH) formed by upregulated expression of hepatic Cyp3a2, responsible for 4βOH formation. Multiple regression analyses identified portal TCA and cecal DCA as positive predictors for liver 4βOH levels. The possible mechanisms linking these predictors and upregulated expression of Cyp3a2 are discussed. Overall, our observations highlight the role of 12α-hydroxylated BAs in triggering liver lipogenesis and allow us to explore the mechanisms of hepatic steatosis onset, focusing on cholesterol and BA metabolism.

3β-Isoobeticholic acid efficiently activates the farnesoid X receptor (FXR) due to its epimerization to 3α-epimer by hepatic metabolism

Bile acids (BAs) are important signaling molecules acting via the farnesoid X nuclear receptor (FXR) and the membrane G protein-coupled bile acid receptor 1 (GPBAR1). Besides deconjugation of BAs, the oxidoreductive enzymes of colonic bacteria and hepatocytes enable the conversion of BAs into their epimers or dehydrogenated forms. Obeticholic acid (OCA) is the first-in-class BA-derived FXR agonist approved for the treatment of primary biliary cholangitis. Herein, a library of OCA derivatives, including 7-keto, 6-ethylidene derivatives and 3β-epimers, was synthetized and investigated in terms of interactions with FXR and GPBAR1 in transaction assays and evaluated for FXR target genes expression in human hepatocytes and C57BL/6 mice. The derivatives were further subjected to cell-free analysis employing in silico molecular docking and a TR-FRET assay. The conversion of the 3βhydroxy epimer and its pharmacokinetics in mice were studied using LC-MS. We found that only the 3β-hydroxy epimer of OCA (3β-isoOCA) possesses significant activity to FXR in hepatic cells and mice. However, in a cell-free assay, 3β-isoOCA had about 9-times lower affinity to FXR than did OCA. We observed that 3β-isoOCA readily epimerizes to OCA in hepatocytes and murine liver. This conversion was significantly inhibited by the hydroxy-Δ⁵-steroid dehydrogenase inhibitor trilostane. In addition, we found that 3,7-dehydroobeticholic acid is a potent GPBAR1 agonist. We conclude that 3β-isoOCA significantly activates FXR due to its epimerization to the more active OCA by hepatic metabolism. Other modifications as well as epimerization on the C3/C7 positions and the introduction of 6-ethylidene in the CDCA scaffold abrogate FXR agonism and alleviate GPBAR1 activation.

Dexamethasone-Induced Liver Enlargement Is Related to PXR/YAP Activation and Lipid Accumulation but Not Hepatocyte Proliferation

Dexamethasone (Dex), a widely prescribed anti-inflammatory drug, was reported to induce liver enlargement (hepatomegaly) in clinical practice and in animal models. However, the underlying mechanisms are not elucidated. Dex is a known activator of pregnane X receptor (PXR). Yes-associated protein (YAP) has been implicated in chemically induced liver enlargement. Here, the roles of PXR and YAP pathways were investigated in Dex-induced hepatomegaly. Upregulation of PXR downstream proteins, including cytochrome P450 (CYP) 3A11, 2B10, and organic anion transporter polypeptide 2 (OATP2), indicated PXR signaling was activated after high dose of Dex (50 mg/kg, i.p.), and Dex at 100 μM activated PXR in the dual-luciferase reporter gene assay. Dex also increased the expression of total YAP, nuclear YAP, and YAP downstream proteins, including connective tissue growth factor and cysteine-rich angiogenic inducer 61, indicating activation of the YAP pathway. Furthermore, nuclear translocation of YAP was promoted by activation of PXR. However, hepatocyte proliferation was inhibited with significant decrease in the expression of proliferation-related proteins cyclin D1 and proliferating cell nuclear antigen as well as other regulatory factors, such as forkhead box protein M1, c-MYC, and epidermal growth factor receptor. The inhibitory effect of Dex on hepatocyte proliferation was likely due to its anti-inflammation effect of suppression of inflammation factors. β-catenin staining revealed enlarged hepatocytes, which were mostly attributable to the accumulation of lipids, such as triglycerides. In summary, high-dose Dex increased liver size accompanied by enlarged hepatocytes, and this was due to the activation of PXR/YAP and their effects on lipid accumulation but not hepatocyte proliferation. These findings provide new insights for understanding the mechanism of Dex-induced hepatomegaly. SIGNIFICANCE STATEMENT: This study identified the roles of pregnane X receptor (PXR) and yes-associated protein (YAP) pathways in dexamethasone (Dex)-induced hepatomegaly. Dex induced PXR/YAP activation, enlarged hepatocytes, and promoted liver enlargement with lipid accumulation, such as triglycerides. However, hepatocyte proliferation was inhibited by the anti-inflammatory effect of Dex. These findings provide new insights for understanding the mechanism of Dex-induced hepatomegaly.

Vitamin D Receptor Deletion Changes Bile Acid Composition in Mice Orally Administered Chenodeoxycholic Acid

The vitamin D receptor (VDR) is a nuclear receptor for the active form of vitamin D₃ and also for the secondary bile acid lithocholic acid (LCA). The in vivo role of VDR in bile acid metabolism remains largely uncharacterized. We previously reported that pharmacological VDR activation enhances urinary bile acid excretion, particularly in mice fed chow supplemented with chenodeoxycholic acid (CDCA), which is metabolized to muricholic acid in mouse liver and is also converted to LCA by intestinal bacteria. In this study, we examined the effect of VDR deletion on bile acid composition utilizing VDR-knockout (VDR-KO) mice. VDR deletion did not change total bile acid levels in liver or feces of mice when fed standard chow supplemented with calcium, needed to prevent hypocalcemia in VDR-KO mice. Total bile acid levels in plasma and urine tended to be higher and lower, respectively, in VDR-KO mice. After feeding CDCA-supplemented chow, VDR-KO mice showed decreased hepatic, fecal and urinary total bile acid and CDCA levels compared to wild-type mice. Plasma total bile acids and LCA were relatively high in these mice. These results indicate that VDR deletion influences CDCA metabolism. VDR may play a role in the excretion of excess bile acids.

Bile Acid Signaling in Neurodegenerative and Neurological Disorders

Bile acids are commonly known as digestive agents for lipids. The mechanisms of bile acids in the gastrointestinal track during normal physiological conditions as well as hepatic and cholestatic diseases have been well studied. Bile acids additionally serve as ligands for signaling molecules such as nuclear receptor Farnesoid X receptor and membrane-bound receptors, Takeda G-protein-coupled bile acid receptor and sphingosine-1-phosphate receptor 2. Recent studies have shown that bile acid signaling may also have a prevalent role in the central nervous system. Some bile acids, such as tauroursodeoxycholic acid and ursodeoxycholic acid, have shown neuroprotective potential in experimental animal models and clinical studies of many neurological conditions. Alterations in bile acid metabolism have been discovered as potential biomarkers for prognosis tools as well as the expression of various bile acid receptors in multiple neurological ailments. This review explores the findings of recent studies highlighting bile acid-mediated therapies and bile acid-mediated signaling and the roles they play in neurodegenerative and neurological diseases.